timeline.inspect returns a feature's definition in the shape timeline.edit's set takes, so agents no longer guess field names: timeline.edit had no documented format ({"extent": {"distance": …}} worked, but only by trying). The new read-only command (feature: id or name) returns type, definition (the editable fields and their current values; bookkeeping and large payloads such as sketch geometry and imported STEP text are left out and listed under omitted), inputs (label, parameter name, expression, unit) and the feature's error and warning. timeline.edit's params doc points to it, and the book's Solid features chapter documents the edit format with the fields of the common features. Tests: inspect, edit by the field names it shows, inspect again (extrude, fillet, sketch); no undo step; hostile feature arguments are refused

crates/engine/src/cmd/edit.rs

@@ -35,8 +35,13 @@ pub static COMMANDS: &[CommandSpec] = &[
         .params("feature: id|name → features deleted with it, and features that would fail without it"),
     CommandSpec::new("timeline.suppress", "Suppress Feature", suppress).params("feature: id|name, suppressed?: bool (default toggles)"),
     CommandSpec::new("timeline.edit", "Edit Feature", edit_feature).params(
-        "feature: id|name, set: {fields to change, e.g. {\"extent\": {\"distance\": \"30\"}}} → recomputed: features rebuilt, \
-             restored: features whose earlier result for the same inputs was reused",
+        "feature: id|name, set: {fields to change, merged into the feature's definition as timeline.inspect returns it, \
+         e.g. {\"extent\": {\"distance\": \"30\"}} (extrude), {\"angle\": \"90 deg\"} (revolve); \
+         expressions are text} → recomputed: features rebuilt, restored: features whose earlier result for the same inputs was reused",
+    ),
+    CommandSpec::new("timeline.inspect", "Inspect Feature", inspect_feature).noundo().params(
+        "feature: id|name → type, definition (the fields timeline.edit's set changes, with their current values), \
+         inputs: [{label, param, expression, unit}] (named inputs parameters.change can also set), error, warning",
     ),
     CommandSpec::new("select.set", "Select", select_set)
         .noundo()
@@ -188,6 +193,52 @@ fn merge(dst: &mut Value, src: &Value, depth: usize) {
     }
 }
 
+/// Fields of a stored feature left out of its definition: bookkeeping timeline.edit recomputes, and
+/// the name and suppression, which timeline.rename and timeline.suppress change.
+const NOT_EDITABLE: &[&str] = &["id", "type", "name", "suppressed", "param_names", "edge_refs", "edge_names", "face_names"];
+
+fn inspect_feature(s: &mut Session, p: &Value) -> Result<Value> {
+    let cmd = "timeline.inspect";
+    let id = feature_id(s, p.get("feature"), cmd)?;
+    let f = s.doc.feature(id).ok_or_else(|| bad(cmd, "feature"))?;
+    let mut def = serde_json::to_value(f).map_err(|e| EngineError::Other(e.to_string()))?;
+    let mut omitted = Vec::new();
+    if let Some(o) = def.as_object_mut() {
+        for k in NOT_EDITABLE {
+            o.remove(*k);
+        }
+        // Large payloads that timeline.edit cannot change.
+        let heavy: &[(&str, &str)] = match &f.kind {
+            FeatureKind::Sketch { .. } => &[("sketch", "the sketch geometry (sketch.inspect; edit it with the sketch commands)")],
+            FeatureKind::Import { .. } => &[("step", "the imported STEP text"), ("components", "the imported product tree")],
+            FeatureKind::MeshImport { .. } => &[("meshes", "the imported meshes")],
+            _ => &[],
+        };
+        for (k, why) in heavy {
+            if o.remove(*k).is_some() {
+                omitted.push(json!({"field": k, "what": why}));
+            }
+        }
+    }
+    let inputs: Vec<Value> = f
+        .kind
+        .inputs()
+        .into_iter()
+        .enumerate()
+        .map(|(i, (label, e, k))| {
+            json!({"label": label, "param": f.param_names.get(i).filter(|n| !n.is_empty()), "expression": e, "unit": k.default_unit()})
+        })
+        .collect();
+    let marker = s.doc.marker.unwrap_or(usize::MAX);
+    let rolled_back = s.doc.features.iter().position(|x| x.id == id).is_some_and(|i| i >= marker);
+    let r = s.model.result(id);
+    Ok(json!({
+        "id": f.id, "name": f.name, "type": f.kind.type_name(), "suppressed": f.suppressed, "rolled_back": rolled_back,
+        "definition": def, "omitted": omitted, "inputs": inputs,
+        "error": r.and_then(|r| r.error.clone()), "warning": r.and_then(|r| r.warning.clone()),
+    }))
+}
+
 fn edit_feature(s: &mut Session, p: &Value) -> Result<Value> {
     let cmd = "timeline.edit";
     let id = feature_id(s, p.get("feature"), cmd)?;